The present invention is related to purification of molten metal. More particularly, the present invention is directed to the removal of hydrogen gas and insoluble impurities from molten aluminum.
Hydrogen is the only gas with significant solubility in molten aluminum. The solubility of hydrogen in molten aluminum is illustrated in FIG. 1. As the temperature of molten metal decreases to the solidification temperature the solubility of hydrogen drops significantly. This significant drop results in the formation of undesirable micro-shrinkage and porosity in the final solidification structure. As indicated in FIG. 1, about 5% of the hydrogen in the molten aluminum remains after completion of the solidification. The remaining 95% is rejected into the liquid until the concentration reaches the point where a hydrogen gas bubble is formed.
Contact of molten aluminum with ambient water moisture is nearly unavoidable under reasonable manufacturing conditions. Unfortunately, molten aluminum is highly reactive and can easily reduce, or decompose, any water present by the reaction:H2O(g)+⅔Al(1)⅓Al2O3+2H
The removal of hydrogen down to an acceptable level prior to solidification is required to obtain a metallurgically sound ingot or casting. The industry accepted practice to remove or lower the dissolved hydrogen content is to bubble an inert or semi-inert purging gas directly through the molten aluminum prior to casting and solidification. The technology related to purging molten aluminum with inert gas is exemplified in U.S. Pat. No. 5,340,379.
Hydrogen dissolved in molten aluminum exhibits a high vapor pressure relative to common alloying constituents and impurities. Therefore, hydrogen can be preferentially removed by purging with inert gas or by vacuum treatment. Hydrogen dissolved in molten metal is removed by the recombination of molecular hydrogen to form hydrogen gas based on the following reaction:H=½H2 (gas)
The chemical equilibrium (KH2) of the reaction is a function of the partial pressure (ρ) given by:KH2=ρH21/2
For pure molten aluminum KH2 is given by:Ln(KH)=5869/T+3.282
There are several ways to directly introduce purging gas into molten aluminum to reduce the hydrogen content. A common method includes the use of a simple pipe or lance, a porous plug, a spinning nozzle degasser or a high-pressure nozzle injection. Exemplary references include U.S. Pat. Nos. 5,340,379; 5,660,614; 6,056,803 and references cited therein.
The rate of removal, and the final hydrogen value obtained, is dependent on several parameters such as the metal temperature, thermodynamic solubility, purging gas flow rate, metal flow rate in the case of continuous degassing, furnace size in the case of static degassing, gas removal ratio and bubble size or surface area. For a given purge gas flow rate the hydrogen removal rate is controlled by the bubble size. The finer the bubble size the higher the rate of diffusion and therefore the higher the rate of removal. A simple lance or tube produces a very large bubble size and therefore results in a relatively slow removal rate. The removal rate is improved by introducing the gas through a porous plug or by a spinning rotor that shears the gas stream into fine bubbles. The finer bubble size results in increased contact surface area with an increased transfer rate and slower bubble ascent rate based on the smaller Stoke's diameter.
There are several limitations in using inert gas bubbles to remove hydrogen from molten aluminum. Efficient removal requires the gas bubbles to be relatively small in order to maximize contact surface area. The smallest gas bubbles are typically obtained with a rotary impeller degasser. The degassers are capable of producing very fine bubbles that can remain suspended for a long period of time. As a result rotary impeller degassers are normally installed a relatively far distance from the casting machine in order to allow sufficient time for gas bubbles to separate by flotation. This distance also allows ample time for re-absorption of hydrogen back into the molten aluminum from atmospheric moisture as well as moisture containing refractory contact materials. The lowest achievable hydrogen content is temperature dependent based on hydrogen solubility-temperature equilibrium. The lower the temperature at which the hydrogen removal process is conducted, the lower the final hydrogen content at solidification. Ideally, hydrogen removal should be made just prior to the onset of solidification, which is not compatible with gas purging.
While rotary impeller degassers are sufficient for generating fine bubbles other problems are created by their use. It is known that filtration, utilizing either a deep bed or ceramic foam filter, is required in addition to degassing. These combined systems typically utilize a significant amount of floor space and require that the molten metal be held between casts in one, or both, treatment units. Holding molten metal creates specific problems. First, an external heat source must be employed to maintain the temperature of the molten metal between casts. This requires an elaborate heating system which is a significant capital expense and has an attendant energy consumption which is expensive and variable. Secondly, the treatment unit must be drained and refilled to change the alloy composition. Draining and refilling is a significant drain on resources requiring non-production labor cost, conversion cost, and productivity losses due to the equipment downtime required for the transition. A compact degasser has been described in P. D. Waite, “Improved Metallurgical Understanding Of The Alcan Compact Degasser After Two Years Of Industrial Implementation In Aluminum Casting Plates”, Conference Proceedings at the 127th TMS/AIME Annual Meeting, San Antonio, February 1998, pages 791-796. This system, while fully drainable, is not compact by current standards. The system also requires substantial ancillary support equipment for the launder including a degassing hood, baffle plates, drive modules including rotors, lifting mechanism, fume exhaust system, PLC panel and interface/gas mixing panel.
A particular problem with the prior art methods of degassing aluminum is the difficulty associated with monitoring the efficiency of the degassing operation. It is well known that a system which can not be effectively monitored can not be optimized for performance.
Summarily, the art has been lacking a suitable degassing and filtering system and apparatus.